First off, its gotta be something which will blow away any 4stroke I might meet off-road, even if it was a totally hopped up thumper. I mean on the straights and long fireroads, through the gears and top speed so it has to have the power to be up-geared and still out-accelerate any off-road thumper you can name. (Within reason, I'm not talking about it being able to beat some custom bike with a Hayabusa engine stuffed into a dirtbike frame and supercharged, lol).
First choice would probably be the Maico 700, although, I'd be a bit skittish as I've never seen an actual test on the thing compared to anything else. 2nd choice would be a '95 or '96 KTM 550 heavily modified by someone like Dick's Racing, who know how to squeeze every drop of hp and torque out of the motor. 3rd choice would be a KX500 with pretty much the same type of mods made to it as has been done to the Project 250 YZ, Micro Blued and what not. 4th Choice would be an '83 Maico Sand Spider with up to date suspension. Why not all of them, dreaming is free, lol. Throw in a KTM or Gas Gas 300 enduro for all-around work and maybe a good 300 trials bike as well, but only after I had at least one of the big boys mentioned above for my high-speed desert riding and sand-hill climbing obsessions.